Design Considerations for WFI Distillation Systems Part 2
Design Considerations for Water for Injection (WFI) Distillation Systems for Improving Quality, Project Performance, and Equipment Life Cycle Cost Reduction was featured in the September/October 2015 issue of Pharmaceutical Engineering® magazine.
This four-part series presents and discusses a number of key requirements and design, quality, and engineering considerations that have high importance in end-user usability, cost control and end-product quality that help manage risks in Water for Injection production and processes. Part two reviews:
- WFI hot or cold loop or storage
- Temperature of feed and cooling water
- How the WFI distillation systems integrate with the storage tank and loop
WFI Hot or Cold Loop or Storage for WFI Distillation Systems
When comparing WFI production equipment to the required storage temperature in a cold loop (20 °C to 30 °C) or hot loop (80 °C or higher), there is a difference in the energy consumption and the selection of the type of WFI production equipment. The benefit of a low-temperature output WFI system, such as a vapor compression system, is lower energy consumption. This type of system is not widely available for all applications. The hotloop and tank WFI applications benefit from WFI supplied at an already high-temperature output of 85°C or even higher that is produced by multiple-effect distillation. Low-temperature output distillation systems are typically intended for immediate use without storage, and these systems require periodic sanitization at high temperatures to reduce bioburden.
Establish a solid and realistic calculation for the equipment based on realistic utility costs, performance values, and available quantities and conditions. Always compare apples to apples.
The best way to estimate these utility costs is by cooperating with utility design engineers and end users of the equipment. Knowing the goal truly helps in making the right decision about the type of equipment needed for the facility today and in the event of a future expansion. Be sure to include the realistic washing, cleaning, and service and maintenance costs at realistic intervals. In addition to the running costs, the service costs play a significant role in the entire life cycle. There is always a risk of malfunction when it comes to complex processes and numerous moving parts; critical part maintenance and replacement costs can be significant, and production downtime is very expensive.
Temperature of Feed Water and Cooling Water for WFI Distillation Systems
For multiple-effect units, feed-water temperature has a major impact on the required flow rates of cooling water. Additionally, it is required to consider the size of heat exchangers in order to work with higher-temperature cooling water. For example, inlet temperatures can be up to 35°C in southern regions compared to 5°C to 10°C in the northern regions. The higher inlet temperatures of feed water and cooling water to the WFI system lead to a smaller temperature differential in the cooling water available for heat transfer. This impacts the flow rate of cooling water, as colder water has more temperature differential. It’s important to consider whether the cooling water supply temperature has seasonal variations, as this may cause other considerations for the capability of controlling the process under different conditions.
Multiple-effect distillation systems need to be evaluated to determine how many effects are optimal for utility use as well as the overall energy consumption of the respective vapor compression system. Individual and project-specific parameters should define the most appropriate system. Include the round-table review of the entire life-cycle cost of equipment, which encompasses energy consumption over the target operational life of equipment (typically 15 to 20 years), service costs, and investment cost.
In today’s world of increasing energy costs and sustainability concerns, it is advised to look at multiple-effect distillation units with enough columns, or any other means of heat recovery, to save in heating- and cooling-water costs. In an MWS, for example, six to eight effects typically means that little to no cooling water is needed to produce WFI and save the most in heating costs, as an increasing number of effects significantly reduces the consumption of heating steam.
How the WFI Distillation Systems Integrate with the Storage Tank and Loop
Communication signals between the WFI tank and the distillation equipment can provide a proportionally controlled capacity. This means that the distillation equipment can automatically adjust to the current demand of WFI consumption based on the direct demand (level of the WFI storage tank). To achieve proportional capacity control, the water still needs to be equipped with a proportional control valve or valves for plant steam and feed-water control, utility measuring instrumentation, as well as a PLC control sequence for running the operation automatically. Feed water can also be controlled by motor inverter control. Eliminating the starts and stops of the WFI still also reduces the time and energy spent on the running up, cooling down, or sanitization period of the units as per the current capacity needs. It may be difficult to think of these savings in numbers, but there is a way: Simply evaluate the number and duration of running cycles and peaks of WFI consumptions put in the simple spreadsheet of a daily schedule. Instead of five daily starts and stops, there may only be two, for example, as the distillation capacity is adjusted according to consumption. It is surprising how much time, energy, and money can be spent annually on ramping up and down the system. It can easily be proved that the payback time of proportional capacity control that has been implemented in a new or existing WFI distillation unit is short enough to justify investment. (See Table A.)
By: Juha Mattile, Sr. Product Manager for Steris Finn-Aqua High Purity Water & Team, VHP Sterilization and Effluent Decontamination systems and Mike Parkka, Project Manager for Steris Finn-Aqua
Catch up on the parts you missed:
- Design Considerations for WFI Distillation Systems Part 1
- Design Considerations for WFI Distillation Systems Part 3
- Design Considerations for WFI Distillation Systems Part 4